In a mold used for manufacturing a composite material structure of the present invention, first segments and second segments each having a plate shape and a convex-shaped obverse surface are each fastened at its end surface to support rings one by one, with the obverse surface being at an upper side, and adjacent segments are joined together at side surfaces thereof, to retain them as a cylindrical mold. In this configuration, in the mold having a substantially cylindrical shape and being dividable into a plurality of segments, the segments are assembled in the cylindrical shape with high position accuracy and in a simplified manner.